A Cosmic Arrow Pierced Pluto's Heart — Is It Still There Beneath the Surface?

When NASA's New Horizons mission sent back its first close-ups of Pluto in 2015, it revealed a giant, heart-shaped basin that dominated the dwarf planet’s geology. Named Tombaugh Regio after Pluto’s discoverer, this “heart” is not a single geological feature but rather two distinct regions. The western lobe, called Sputnik Planitia, is a vast, elongated depression, as large as a quarter of the United States. It’s filled with bright white nitrogen ice that sits several kilometers lower than the surrounding terrain.

Scientists believe this striking basin formed billions of years ago when a massive object slammed into the dwarf planet. Now, a team of scientists from the University of Bern in Switzerland and the University of Arizona has used computer simulations to dig deeper into the origins of Sputnik Planitia, finding that a large chunk of the impactor could still be buried beneath the nitrogen ice. Their research, published April 15th in Nature Astronomy, also challenges previous assumptions about Pluto's internal structure, hinting that a long-suspected subsurface ocean is absent.

Shortly after Pluto’s flyby, scientists noticed that Pluto’s “heart” was located at a special location, close to the equator and right on its tidal axis. “If you drew a line from Pluto to its neighboring Moon, Charon, which it's tidally locked to, that line goes right through the heart of Pluto,” says James Keane (Jet Propulsion Laboratory), who wasn’t involved with the new study. In two independent papers published in 2016, Keane and other researchers proposed that Sputnik Planitia likely didn’t form at the same latitude where it is today, but Pluto likely rotated on its axis to accommodate a large amount of extra mass beneath the ice at the lowest energy point for the system, a process known as true polar wander. The 2016 papers proposed that the extra mass likely came from a subsurface ocean under Sputnik Planitia.

The Impact

In the new research, the team used 3D simulations that could account for the angle of the impact, which was already expected to have occurred from an oblique angle due to the tear-shaped form of the basin. Previous work had only accounted for 2D, head-on impacts. The new simulations also took into account the physical strength of the materials, which, it turns out, play an important role if the impact occured at a relatively low velocity. This was very likely the case for this impact, due to Pluto’s low gravity.

“Immediately, when I ran the simulations, it was different to what one would expect,” says Harry Ballantyne (University of Bern, Switzerland), who led the study. Pluto’s small mass and weak gravity allow for a much slower impact, he explains. “So, instead of it being super destructive and melting so much, everything stays solid in the entire process. Also the rock of the impactor is way, way, way below the melting temperature of rock, so it can just remain intact as one thing the whole time.”

By meticulously varying the composition and size of the impactor, along with its incoming velocity and angle, the team concluded that an impactor was roughly one-third the size of Pluto, about 700 kilometers (400 miles) in diameter, with a rocky core covered by an icy mantle.

During the impact, the simulations show, the impactor creates a crater that pushes away Pluto’s primordial ice. The core of the impactor, which remains mostly intact due to its material strength, dives in toward the core. But instead of driving through, it connects with the core and almost bounces off of it.  It spreads into a splat right at the core-mantle boundary.

In the meantime, the impactor’s icy mantle spreads to fill the cavity created by the impact. So the entire region is almost all impactor material.

By preserving the rocky core of the impactor beneath the ice, the impact itself could have created an excess of mass, not a deficit as previously thought. As a result, Pluto’s spin would have oriented to put this region at the equator — eliminating the need for a subsurface ocean to accomplish the same feat.

“I think that this is an entirely valid hypothesis,” Keane says. However, he points out that the new research does little to test if the impactor’s mass is enough to justify the true polar wander of Pluto. “They leave that to future work.”

A Warmer Pluto?

But what if the impactor had found a planet that wasn’t as thoroughly frozen as Pluto is today? The impact that formed Sputnik Planitia likely happened early in Pluto’s history, and there are reasons to think the planet could have been warmer back then, argues William McKinnon (Washington University in St. Louis), who wasn’t involved with the current study. And a warmer Pluto would have had a subsurface ocean.

Currently, Pluto has a rocky core and an ice mantle, but that layering suggests a warmer past “You don't get such a structure unless you heat the body up enough that at least water ice will melt, and the rock falls to the center,” McKinnon explains. That warmth would have come from remnant heat from the world’s formation as well as radioactive decay in its core.

“From my point of view, [the study’s findings are] not self-consistent with the evidence we have from Pluto's long history, in which the surface has been broken up in what we call extensional tectonic features,” McKinnon says. Those features suggest that a subsurface ocean existed at some point, and slowly expanded in size as it cooled and froze. “Because ice is less dense than water, it means that the landscape of Pluto has been slowly rising and therefore splitting apart.”

“It's obviously an interesting point,” Ballantyne answers; however, he adds, “We did explore some different temperature profiles, and basically what we find is that this mechanism doesn't seem to be ultra-sensitive to temperature.” The only important thing, Ballantyne says, is that it’s cold enough so “everything remains solid.”

Outer-Solar System Impacts

Giant impacts might just work differently in the ultracold, small bodies that float in the solar system’s outskirts, Ballantyne says. Maybe, regions dominated by impactor material are common on these worlds, and weird blobs of rock sitting at their core-mantle boundaries are the norm. “There's something very interesting about impacts in icy dwarfs, full stop,” Ballantyne says.

“Pluto showed us just how interesting Kuiper Belt objects are,” Keane says. “This is a great example of showing how profound the New Horizons mission, and Pluto in particular, really was.”